Update Dependencies.scala

update async-http-client to 1.7.21
Remove deprecated code from Promise, F.Promise and Akka.
Merge pull request #1851 from richdougherty/rm-deprec

Remove deprecated code from Promise, F.Promise and Akka.
Add missing import

It's necessary to have the play.Keys in scope for this to work
Merge pull request #1843 from hepin1989/master

Update Dependencies.scala
Merge remote-tracking branch: upstream/master